******************************* PRESS RELEASE *****************************
CONTACT:
Dr. William J. Merline
(303) 546-0487 or merline@boulder.swri.edu
Maria Martinez
(210) 522-3305 or mmartinez@swri.org
EMBARGOED FOR RELEASE UNTIL OCTOBER 6, 1999 AT 2:00 PM EDT
ASTRONOMERS DISCOVER MOON ORBITING ASTEROID
New technology promises to revolutionize our understanding of asteroid makeup
Boulder, Colorado. October 7, 1999. An international team of astronomers has
discovered a moon orbiting the asteroid (45)Eugenia. The pictures, taken with
the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, are the first
images of an asteroidal satellite taken from Earth. The team's findings will
be reported in the October 7 issue of Nature.
Previous attempts to photograph such satellites, using both ground-based
telescopes and the Hubble Space Telescope, found no satellites. The only other
such picture came from an interplanetary spacecraft, Galileo, when it
discovered the small moon, now known as Dactyl, around asteroid (243)Ida
in 1993. The observations could only be accomplished because of a new
technique, called adaptive optics, that reduces the blurring caused by the
Earth's atmosphere.
A surprising result of this discovery is the very low density of the primary
asteroid -- only about 20 percent denser than water. Most asteroids appear
dark and were thought to be composed primarily of rock, which is about three
times denser than water. "A picture is emerging that some asteroids are real
lightweights," said Dr. William Merline, leader of the team, and a senior
research scientist at the Boulder office of San Antonio-based Southwest
Research Institute (SwRI). A recent flyby of the NEAR spacecraft confirmed
that another asteroid, (253)Mathilde, also has a low density. "Either these
objects are highly porous rubble-piles of rock, or they are mostly water
ice," said Dr. Clark Chapman, another team member, also from SwRI.
The presence of a moon allows scientists to determine the mass of an asteroid
because of the effect of the primary asteroid's gravity on its small moon. The
size of most asteroids is known from standard astronomical studies. If both
the mass and the size are known, researchers can learn the asteroid's density.
The density then gives a clue to the asteroid's makeup -- either in terms of
composition or structure.
"If these asteroids are rubble-piles, it tells us about the severity of
collisions in the asteroid belt and its subsequent evolution. If the objects
are largely ice, covered with a dark-coating, then these objects may be
remnants of burned-out comets and will further our understanding of the
connection between comets and asteroids," said Dr. Christophe Dumas of the Jet
Propulsion Lab in Pasadena.
"It is almost certain that the satellite was formed by a collision," said
Merline. "As we know from the formation of our own moon and the craters on
planetary surfaces, collisions played a large role in the formation of our
solar system. Satellites of asteroids give us a window into these collisions,
and help us understand how and why our solar system looks like it does."
The light from stars and other celestial objects is distorted by the
atmosphere, much as water distorts our view of an underwater object. The new
technique, pioneered at the University of Hawaii by team member Dr. Francois
Roddier, analyzes the distortions and corrects the light beam by means of what
is essentially a "fun-house mirror" back into its previous, undistorted form.
"CFHT's exceptional site, telescope, and adaptive optics now allow us to see
far sharper detail through the Earth's atmosphere. In many cases we can now
compete with the clarity of space-based telescopes," said Roddier. The
instrument used was built by the CFHT Corporation.
Previously, faint and close satellites would have been lost in the glare of
the primary asteroid. "It is similar to taking a photo of a candle located 400
km away and then discovering a firefly (that is 300 times fainter) flying
within two meters of the flame," said Dr. Laird Close, a participant from the
European Southern Observatory (ESO) in Germany.
The results are the first from a program to search for satellites around
nearly 200 asteroids. "If more satellites are found, it will revolutionize our
understanding of the makeup of asteroids," said Merline.
"Except for a few of the very largest asteroids, this is the only way that
asteroid densities can be determined other than by spacecraft flybys,"
according to Close.
Eugenia orbits the sun in the main asteroid belt, a collection of thousands of
asteroids that exists between the orbits of Mars and Jupiter. Asteroids are
thought to be bodies that never formed a planet; the gravity of the giant
planet Jupiter may have stirred up the bodies enough that they collided with
each other at fast speeds, perhaps either fragmenting or forming satellites,
rather than colliding gently, adhering, and gradually building up a planet.
Researchers estimate that the diameter of the satellite is about 13 kilometers.
Eugenia's diameter is about 215 kilometers. The researchers have determined
that the satellite has a circular orbit about 1,190 km away from Eugenia. It
orbits about once every five days.
While awaiting assignment of a permanent name, the satellite has been given
provisional designation, by the International Astronomical Union, of
S/1998(45)1, the first satellite of asteroid (45) that was discovered during
1998.
This work was funded by NASA and the U.S. National Science Foundation. A
portion of the image processing and data analysis was carried out using
facilities at the ESO. CFHT is funded by the National Research Council (NRC)
of Canada, the Centre National de la Recherche Scientifique (CNRS) of France,
and the University of Hawaii.
Other team members and affiliations are Dr. Francois Menard, CFHT; Dr. David
Slater, SwRI headquarters in San Antonio; Dr. Gilles Duvert, Laboratoire
d'Astrophysique in Grenoble, France; Dr. Chris Shelton, W.M. Keck Observatory,
Hawaii; and Dr. Tom Morgan, NASA Headquarters, Washington, D.C.
EDITORS: images to support this story are available after 2 PM EDT, Wednesday,
October 6, 1999, on the Internet at:
www.boulder.swri.edu/~merline/press_release
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